The frizzled receptor is a seven-transmembrane receptor belonging to the family of G-coupled receptors, with a long extracellular domain. This extracellular part has a cysteine-rich domain, called the CRD, which defines the binding region for the natural ligands, the so-called Wnt proteins. The intracellular part enables the prolongation of the signal. A schematic representation of the structure of frizzled receptors is shown in FIG. 1.
There are ten different frizzled receptors that slightly differ in the variable cytoplasmatic part and the Wnt binding domain. When an agonistic ligand binds to the receptor, a signal transduction cascade gets activated. The Wnt/frizzled signaling can be subdivided in three pathways: first, the canonical pathway or beta-catenin-dependent pathway with second messenger β-catenin; second, the a-specific or beta-catenin-independent pathway formerly known as non-canonical pathway via calcium; and third, the planar cell polarity pathway.
There are 19 different Wnt proteins that can be subdivided into two families, namely, the Wnt1 class, inducing secondary axis via the canonical pathway and the Wnt5a class working via the second messenger calcium, without inducing secondary axis.
Wnt proteins are very large proteins that tend to stick to the extracellular matrix and other biological and non-biological substances in a non-specific way. Because of its lack of specific binding, the natural ligand of the frizzled receptor is not suitable for visualization of the receptor. Native Wnt proteins may be experimentally used, however, for the induction of the Wnt signaling pathway through interaction with a frizzled receptor.
Many developmental diseases and diseases linked with a reactivation of the embryonic gene program are associated with re-expression of the Wnt/frizzled signal transduction cascade. The Wnt/frizzled pathway provides screening possibilities, not only for wound healing in the heart, but also for osteo-arthritis and rheumatic arthritis. Main differences between these two are based on the presence or absence of the frizzled receptor. Specific frizzled binding peptides may, therefore, differentiate between the two and make more specific and appropriate treatment of these patients possible. Furthermore, diseases as idiopathic pulmonary disease, liver disease and renal fibrosis, are associated with the Wnt/frizzled pathway. All these diseases give rise to a reactivation of the embryonic gene program, therefore, activation of the Wnt/frizzled signal transduction cascade. This reactivation will lead to fibrosis and, therefore, organ malfunctioning.
Proteins from the Wnt family have been implicated in cell—cell communication in a wide variety of developmental and physiological processes. Wnt signaling is required for different aspects of cardiac and vascular development, including myocardial specification, cardiac morphogenesis and cardiac valve formation, as well as endothelial and vascular smooth muscle cell proliferation. Defective Wnt signaling can result in different cardiac and vascular abnormalities.
In the adult heart and blood vessels, Wnt signaling activity is quite low under normal conditions. However, this pathway is reactivated during the pathological remodeling induced by pressure overload, in injured arteries and after myocardial infarction.
Myocardial infarction (MI) is characterized by the death of cells in the heart due to occlusion of a coronary artery, which supplies blood to the heart. Some people will have a relatively good functioning heart after MI, whereas others have dilated hearts, which function very badly.
It has been described that the well-healed heart contains more myofibroblasts in the infarcted area. Further research has shown that these hearts have increased levels of frizzled-1 and, especially, frizzled-2; whereas these receptors were mainly present on newly formed myofibroblasts. It has been proposed that these myofibroblasts give the heart the ability to preserve some of its geometry and structure.
Genetically modified animal models have shown that inhibition of Wnt signaling results in increased angiogenesis, better infarct healing and an attenuated hypertrophic response of the heart. This suggests that pharmacological inhibition of Wnt signaling could provide a good therapeutic strategy to prevent excessive cardiac and vascular remodeling (van de Schans et al., Eur. J. Pharmacol. 585:338-345 (2008)).
Antagonist of the Wnt/frizzled pathway may, therefore, prevent fibrosis and prevent malfunctioning of vital organs as the heart (Blankesteijn et al., 1996; van de Schans et al., 2008), lungs (Konigshoff et al., 2008), kidney (Surendran et al., 2002) and liver (Thompson and Monga, 2007). Antagonists of the Wnt/frizzled pathway may also be useful in the treatment of injured skeletal muscles, which could be better healed after inhibiting the Wnt/frizzled signal transduction. Aging may also be slowed down by inhibition of the Wnt/Fz signal transduction cascade. (Brack et al., 2007; Imai et al., 2006; Konigshoff et al., 2008; Li et al., 2004; Surendran et al., 2002; Thompson and Monga, 2007.)
Unfortunately, no antagonists for the frizzled receptors have been described to date. Therefore, until now, the only way to intervene in the Wnt/frizzled pathway is at the level of the second messenger or other downstream signal elements, leading to aspecific blocking and interference in other signaling transduction cascades.
To limit the influences on other biological processes, it would be desirable to intervene on the receptor-ligand level by providing antagonists for the frizzled receptor.